1. Field of the Invention
The present invention relates to a visual axis detector, and more particularly to a visual axis detector which detects a direction of gazing point (visual axis) along which an observer (photographer) observes on an observation plane on which an object image is formed by a lens system of an optical device such as a camera, by utilizing a reflection image which is formed when an eyeball plane of the observer is illuminated.
2. Related Background Art
Various types of visual axis detectors for detecting the viewing direction (visual axis) along which the observer observes on the observation plane have been proposed.
For example, in Japanese Laid-Open Patent Application No. 61-172552, a light beam from a light source is directed to a front eye portion of an eye-ball under test and the sight axis (gazing point) is determined by making use of a focusing state of a reflection image due to a reflected light from a cornea and an iris.
FIG. 6 illustrates the visual axis detection method proposed in the above Patent Application, with a modification for illustration purpose.
In FIG. 6, numeral 104 denotes a light source such as a light emitting diode which emits an infrared ray which is insensitive to the observer. It is arranged on a focus plane of a projection lens 106.
The infrared ray emitted from the light source 104 is collimated by the projection lens 106, reflected by a half-mirror 110 and illuminates a cornea 1 of an eyeball 101. A portion of the infrared ray reflected by the surface of the cornea 1 passes through the half-mirror 110 and is focused by a lens 107 to a position d' on an image sensor 109. Symbol d represents a first Purkinje's image which is a reflected virtual image of the light source by the cornea. Ends a and b of the iris 3 are focused to positions a' and b' on the image sensor 109 through the lens 107. When a rotation angle .theta. of an optical axis B of the eyeball relative to an optical axis A of the lens 107 is small, a coordinate Zc of a center position c of the iris 3 is represented by ##EQU1## where Za and Zb are coordinates of the ends a and b of the iris 3.
The rotation angle .theta. of the optical axis B of the eyeball substantially meets a relationship of EQU OC.multidot.sin .theta..apprxeq.Zc-Zd (1)
where Zd is a Z-coordinate of the position d at which the first Purkinje's image appears, and OC is a distance from a center of curvature 0 of the cornea 1 and a center C of the iris 3. Thus, the rotation angle .theta. of the optical axis B of the eyeball can be determined by detecting the position of each of the peculiar points (the first Purkinje's image position d and the ends a and b of the iris) projected on the image sensor 109. The formula (1) may be modified to ##EQU2## where .beta. is a magnification determined by a distance l.sub.1 between the position d of the first Purkinje's image and the lens 107, and a distance l.sub.0 between the lens 107 and the image sensor 109. Normally, it may be regarded as a substantially constant value.
By incorporating a device which detects the visual direction (gazing point) of the eye of the observer under test into a finder of a one-eye reflex camera, it is possible to determine which position on a pint glass the photographer is observing.
This is advantageous in an automatic focus detector having distance measurement points not only at the center of field but also at a plurality of points in the field. Namely, when the observer attempts to make the automatic focus detection by selecting one of the distance measurement points, a process to input the selected one point may be omitted but the point which the observer is observing is regarded as the distance measurement point so that the distance measurement point is automatically selected to effect the automatic focus detection.
However, when the visual axis detector proposed in the Japanese Laid-Open Patent Application 61-172552 is applied to detect the visual axis of the observer who observes through the finder of the camera, there is a problem if a working distance between the visual axis detection optical system comprising illumination means and photo-detection means, and the eyeball of the observer is not constant but variable. For example, assuming that the distance l.sub.1 between the lens 107 and the position d of the first Purkinje's image in FIG. 6 is not a predetermined value but different from that value, the magnification .beta. defined by the formula (2) is not constant. As a result, an error is induced in the calculation of the rotation angle .theta. of the optical axis of the eyeball.
As a result, the observer must make a manual calibration so that the gazing point of the observer matches to an index indicated by the visual axis detector. This is very troublesome.
Another method for detecting the visual axis is described in JOSA Vol. 63, No. 8 p.921 et seq, and U.S. Pat. No. 4,574,314 discloses a technique to apply the detection of the eye movement to an auto-focusing system of a television camera.
A related technology is described in U.S. patent application Ser. No. 406,588 assigned to the present assignee.